Thick and wide strip material, e.g. steel strip, is reduced in size, and its strength and flexibility are increased by moving it in a run consisting of a multiplicity of passes between rolls that compress it with great force. Such action, combined with some tension in the strip, decreases the workpiece thickness and its width, while increasing the length of the strip. (As here intended, the thickness of the workpiece is its smallest dimension as compared to its length which extends in the travel direction and its width which is perpendicular thereto and to the compression direction.) The workpiece travel speed increases slightly with each pass between the rolls.
The rolling is done in roll stands each comprising two parallel and small-diameter working rolls that define a nip through which the strip passes. A pair of large-diameter backing rolls flank these working rolls, each backing roll engaging the respective working roll in line contact and urging it toward the other working roll. Thus the working rolls can exert enormous pressure on the workpiece because of the small contact area.
The natural tendency of this force to bend these small-diameter working rolls outward away from the workpiece is countered by the stiffer large-diameter backing rolls. In addition complex systems are provided to oppositely bend the backing and working rolls so they are convex toward the workpiece to substantially eliminate any bend in the working rolls.
The workpiece width is invariably smaller than the effective width of the rolls, which are cylindrical with stub shafts on their ends so this effective width is the length of the middle large-diameter cylindrical portion. The contact width, that is the length of the contact zone between the workpiece and the working rolls measured parallel to the roll rotation axes which are in a plane perpendicular to the workpiece travel direction, is generally equal to slightly less than the effective roll width for the first pass, which may include several passages through a given roll stand, and is a small fraction of this effective width for the last pass. The disparity between effective roll length and workpiece width therefore increases during the run.
In addition to bending, the working rolls are subject to limited elastic deformation in the form of a flattening in the region of contact with the workpiece. The out-of-contact end portions of the two working rolls are not flattened, but are cylindrical, so the nip between the working rolls is slightly smaller to both sides of the workpiece than at the workpiece. As a result the longitudinal strip edges are subjected to greater pressure and are compressed, a phenomenon known as edge drop. Thus the strip workpiece is not of uniform thickness.
To overcome this problem the above-mentioned roll-bending equipment is used to bring pressure on the working-roll ends. Obtaining enough bend to cancel out the above-described flattening of the rolls is very difficult.
In addition it is known to make the working rolls slightly barrel-shaped so that, when they bend, their side in contact with the workpiece is perfectly straight. The problem with this type of arrangement is that the rolls then are only suitable for use for a limited range of workpiece widths, needing complex remachining for different sizes.
In another known system, such as described in German Pat. No. 955,131 and German ALS No. 2,206,912, the working rolls are braced against outwardly tapered or barrel-shaped intermediate backing rolls in turn backed up by cylindrical rolls. The frustoconically tapered end regions of the intermediate backing rolls start above level with the strip edges. Thus as band width changes the intermediate backing rolls must be changed also, necessitating the use of complex supports for the rolls as well as a magazine of different roll sizes.
A solution to this problem has been the use of intermediate backing rolls which each have only one tapered end region and which can be moved axially in the roll stand. The edge of this end region is aligned vertically with a respective workpiece edge, and is moved in as the rolling operation progresses and the workpiece becomes narrower. Obviously the equipment that does this is extremely complex, expensive, and difficult to operate. In addition such uneven bending of the working rolls creates a workpiece of nonuniform thickness in its central regions. The working rolls in such a system also wear at an excessively fast rate.
In copending and commonly owned patent application Ser. No. 469,137 filed Feb. 23, 1983 by H. Feldman et al, now U.S. Pat. No. 4,479,374 issued Oct. 30, 1984, a rolling method is described in which the diameters of the projecting ends of the working rolls are reduced by removal of material from them several times after respective passes in a single run so that the working rolls only contact the respective backing rolls along a distance equal generally to the workpiece width.
This decrease in effective roll width as the workpiece width decreases during a run takes the end portions of the working rolls out of contact with the backing rolls. The center portions of the working rolls are meanwhile flattened somewhat both on the side in contact with the workpiece and diametrally opposite thereto on the side in contact with the respective backing rolls. The end region of each working roll is not in contact with the workpiece, however, so it is not flattened on this side. Similarly since these end regions are of smaller diameter than the rest of the central part of the rolls they do not touch the backing rolls either and are not flattened on this side.
If these end regions were not cut away they would contact the backing rolls which would therefore bow the working rolls somewhat toward each other, causing excessive compression of the workpiece at the edge, so-called edge drop. Periodically reducing the effective width of the working rolls symmetrically to a central plane longitudinally bisecting the workpiece and perpendicular to the working-roll axes eliminates this effect. In use the working rolls might still bend slightly and contact the backing rolls at their cut-in end regions, but even so the finished workpiece will exhibit virtually no edge drop.
This procedure is still relatively laborious, as it normally is necessary to take the rolls off the stand to machine each time the diameter is to be reduced. Even if the material-removing machinery, such as a grinder, is mounted right on the roll stand, its operation is usually difficult, in particular with respect to dimensional accuracy.